A motor apparatus of the aforementioned type is used for example for rotating an optical disc in an optical disc apparatus by means of such a motor as spindle motor, and known motor apparatuses are disclosed for example in Japanese Patent Application No. 2004-019043 and Japanese Patent Laying-Open No. 2002-218783 (Patent Document 1). Such a conventional motor apparatus is shown in FIG. 4. This motor apparatus 101 includes a motor 102, a motor driver 107, a drive-current-detection resistance element 108, and a motor drive control circuit 106. Motor driver 107 supplies drive current to armature coils LU, LV, LW of motor 102 for driving motor 102 and controlling the rotational speed. Drive-current-detection resistance element 108 through which this drive current flows accordingly generates a motor-drive-current detection voltage proportional to the drive current. Further, a motor control command unit (not shown) configured with a CPU for example receives from a rotational speed counter 104 information about the rotational speed of motor 102 and outputs a command voltage VCM to motor drive control circuit 106 so that motor 102 rotates at a desired rotational speed. Motor drive control circuit 106 controls, as described hereinlater, motor driver 107 using this command voltage VCM and the aforementioned motor-drive-current detection voltage.
Motor drive control circuit 106 is now described in detail. Command voltage VCM from the motor control command unit is input via a control terminal CNT to a control voltage conversion circuit 110. Control voltage conversion circuit 110 generates a voltage (rotational-speed control voltage) VRC proportional to a difference between command voltage VCM and a predetermined reference voltage VREF, relative to a ground potential. A peak hold circuit 114 holds a peak voltage (detected peak voltage) VDETP of the motor-drive-current detection voltage from drive-current-detection resistance element 108. A rotational-speed control amplifier 115 compares rotational-speed control voltage VRC with detected peak voltage VDETP to amplify the difference and output the amplified one. Circuits in the subsequent stage of the above-described circuits control motor driver 107 so that detected peak voltage VDETP follows to be identical to rotational-speed control voltage VRC. This peak hold circuit 114 is comprised, as shown in FIG. 5 for example, of a differential amplifier circuit 151, an NPN-type transistor 152, a capacitor 153, and a resistance element 154, and holds at an output terminal OUT the peak voltage of the motor-drive-current detection voltage at an input terminal IN.
FIG. 6 illustrates characteristics of the rotational speed of motor 102 with respect to command voltage VCM from the motor control command unit. Between a rotational speed of T0 (e.g. 100 rpm) and a rotational speed of T1 (e.g. 10000 rpm), the rotational speed is substantially linear with respect to command voltage VCM. Therefore, T0 and T1 are respectively the minimum rotational speed and the maximum rotational speed that can be controlled and thus generally the rotational speed is controlled between T0 and T1. Here, T0 is determined for example by a minimum detected peak voltage VDETP that can stably be held by peak hold circuit 114 and by noise immunity.
Patent Document 1: Japanese Patent Laying-Open No. 2002-218783